The present invention is directed to delivery of a glass stream for forming glass charges for glassware manufacture, and more particularly to a method and apparatus for delivering a so-called cased glass stream in which an inner or core glass is surrounded by an outer or casing glass.
It has heretofore been proposed to provide a cased glass stream for forming glassware having layered wall segments. U.S. application Ser. Nos. 08/374,371 and 08/374,372 disclose techniques for delivering such a cased glass stream in which core glass from a first source is delivered through a first orifice. A second orifice is vertically spaced beneath and aligned with the first orifice, and is surrounded by an annular chamber that communicates with the second orifice through a gap between the first and second orifices. A heated tube delivers casing glass from a second glass source to the annular chamber that surrounds the second orifice. Glass flows by gravity through the first and second orifices from the first and second sources in such a way that a cased glass stream emerges from the second orifice. This cased glass stream may be sheared by conventional techniques to form individual cased glass gobs for delivery to conventional individual section glassware forming machines.
Although the techniques disclosed in the noted patent applications address and overcome problems theretofore extant in the art, further improvements remain desirable. For example, the presence of air bubbles, sometimes termed xe2x80x9cblisters,xe2x80x9d in the casing glass stream has been a problem. Flow of casing glass from the casing glass spout is controlled by a spout tube which is positioned over the casing glass spout outlet opening so as to meter casing glass flow at the desired volumetric ratio relative to core glass flow. However, the volumetric ratio of casing glass flow to core glass flow is very low, such as on the order of 5 to 10%. Consequently, when using conventional glassware forming equipment, the extremely low volume of casing glass flowing through the casing glass spout outlet forms a thin fall, around one-quarter inch thick, around the outlet opening and around the upper portion of the heated delivery tube, with the volume within this fall being open. After a period of operation, air bubbles or blisters begin to appear in the casing glass stream. It is believed that a chimney-like effect of the heated air within the interior of the spout outlet opening and the interior of the casing glass delivery control tube creates a pressure differential or gradient between the ambient atmosphere outside of the casing glass spout and the interior within the thin glass fall. It is believed that this pressure gradient promotes migration of air through the refractory material of the casing glass spout, and eventually into the thin glass fall within the spout outlet.
A number of techniques have been proposed in an effort to eliminate this air bubble or blister problem, including lining of the spout outlet opening with platinum in an effort to block air migration. The technique that is currently preferred is periodically to xe2x80x9cfloodxe2x80x9d the casing glass outlet and heated delivery tube with casing glass far in excess of that needed for forming the cased glass stream, and to maintain this excessive casing glass flow for a period of time. It is believed that this xe2x80x9cfloodingxe2x80x9d of the casing glass delivery path eliminates the chimney effect previously described, and further that hydrostatic pressure on the casing glass promotes flow of casing glass into the refractory material of the spout outlet opening so as to block air migration paths. When casing glass flow at reduced level is resumed, the air bubbles or blisters are eliminated for a period of time. However, continued use of the ceramic spout requires that the described xe2x80x9cfloodingxe2x80x9d operation be undertaken with increasing frequency, apparently due to increasing erosion and wear of the spout material. It is believed that, as the refractory spout material ages, it becomes more difficult to fill the air migration cracks and passages within the casing glass spout. In any event, the described xe2x80x9cfloodingxe2x80x9d operation detracts from glass production, and therefore undesirably increases production costs. Furthermore, production of cased glass having air bubbles or blisters in the casing layer results in undesirably increased scrap rates, further increasing production costs.
It is therefore a general object of the present invention to provide a method and apparatus for delivering a glass stream, particularly a cased glass stream, in which formation of air bubbles or blisters in the thin glass fall of the casing glass stream is reduced or eliminated, and in which the need periodically to xe2x80x9cfloodxe2x80x9d the glass stream delivery path is also eliminated. Another and related object of the present invention is to provide a method and apparatus for delivering a glass stream, particularly a cased glass stream, that is characterized by improved production efficiency and therefore reduced manufacturing cost as compared with similar prior art techniques.
Apparatus for forming a cased glass stream having an inner core glass surrounded by an outer casing glass in accordance with a presently preferred embodiment of the invention includes a spout for delivering core glass from a first source through a first orifice. A second orifice is vertically spaced beneath and aligned with the first orifice, and is surrounded by an annular chamber that communicates with the second orifice through a gap between the first and second orifices. A tube delivers casing glass to the annular chamber from the outlet opening of a casing glass spout in such a way that glass continuously flows by gravity through the orifices from the first and second sources to form the cased glass stream. A hollow spout tube within the casing glass spout is positioned with respect to the spout outlet opening for metering flow of casing glass through the outlet opening and delivery tube to the annular chamber surrounding the orifices. The interior of this spout tube is coupled to a source of gas under pressure so as to maintain the tube interior and the interior of the thin fall of casing glass through the spout outlet opening at a pressure above ambient pressure surrounding the easing glass spout. This elevated gas pressure within the casing glass fall reverses the pressure differential between the interior and exterior of the spout outlet opening, so that any tendency for air to migrate through the refractory material surrounding the outlet opening and into the casing glass fall is eliminated.
In accordance with another aspect of the present invention, there is provided an apparatus for delivering a glass stream that includes a glass spout having a lower outlet opening and a flow control spout tube disclosed within the spout. The spout tube has a closed upper end, a hollow interior and an open lower end adjacent to the spout outlet opening, and position of the tube within the spout is controlled so as to control flow of glass through the outlet opening. The hollow interior of the flow control tube is coupled to a source of gas under pressure so as to maintain the tube interior at a pressure above that of ambient air surrounding the spout. Thus, a third aspect of the invention contemplates a method of preventing permeation of air through the refractory material around the spout outlet opening into the glass flowing through the outlet opening by delivering gas under pressure to the flow control tube so as to maintain gas pressure within the tube and within the spout outlet opening above ambient air pressure around the spout. By way of example, gas pressure within the flow control tube and the spout outlet opening may be maintained at a pressure of about two inches of water column above ambient. The gas maintained under pressure within the spout tube may comprise air, nitrogen or argon. In some applications, such as manufacture of amber glass, it may be desirable to provide a reducing gas within the spout tube and in contact with the glass flow, which would thus advantageously change the nature of the atmosphere in contact with the glass flow. For example, methane or another combustible gas may be injected into the spout tube to burn and maintain a reducing atmosphere at elevated pressure in contact with the glass flow.